Vehicle suspensions

ABSTRACT

A vehicle air suspension system includes a control unit  32  which is arranged to produce a running measure of the level of cross articulation of the two axles  18, 20  and, if it exceeds a certain level indicating that the vehicle is on rough terrain, to open respective interconnections  34, 36  between the suspension units  24  on opposite ends of each axle so as to reduce the resistance to that articulation. The interconnections are arranged to close when the vehicle speed increases so as to provide roll control.

FIELD OF THE INVENTION

The present invention relates to active and semi-active vehiclesuspensions and in particular to such suspensions for off-road vehicles.

BACKGROUND TO THE INVENTION

Vehicle suspensions are characterized in various ways, including thestiffness in roll, single axle articulation, and cross articulation.There are two aspects of a suspension which determine how easily thewheels can move in articulation. One is the damping rate of the dampers,and the other is the spring rate of the springs in the suspension. Theterm stiffness refers to the spring rate. Therefore the articulationstiffness of the front axle of a vehicle is expressed as a force(expressed as a moment or a linear force) per unit of articulationdisplacement of the front wheels (expressed as an angular articulationdisplacement or a linear difference in ride height). The crossarticulation stiffness is defined as a force per unit of crossarticulation displacement, expressed, for example, as a lineardifference in average ride height between two pairs of diagonallyopposite wheels.

When a vehicle is travelling over rough terrain it is desirable to allowa high degree of articulation of the wheels, that is vertical movementof the wheels on opposite sides of the vehicle in opposite directions.In particular a high degree of cross articulation is desirable, that isarticulation of the front wheels in one direction and articulation ofthe rear wheels in the opposite direction. This enables the vehicle tomaintain traction over highly uneven ground. However this can conflictwith the need for firm roll control when the vehicle is travelling athigher speeds on a road, since roll can be considered as articulation ofthe front and rear axles in the same direction.

This conflict arises, for example, in interconnected fluid suspensions,such as interconnected air suspensions, where the level ofinterconnection between air strings on opposite sides of the vehicle canbe selected to control the level of resistance to articulation.

For example, it is also known from U.S. Pat. No. 5,765,115 to provide anair suspension system in which the air springs of the two rear wheelsare interconnected by a pipe, which can be closed and opened by a gatevalve, and the air springs of the two front wheels are similarlyinterconnected. Closing the valves increases roll stiffness of thevehicle, and opening them increases the ease of articulation.

The same conflict arises with the use of anti-roll bars which need to bestiff to provide good roll control on road, but much less stiff to allowsufficient articulation off-road.

This problem has been addressed, for example, by the type of systemdisclosed in U.S. Pat. No. 4,796,911 which discloses a vehicle with asplit anti-roll bar, with a hydraulic de-coupling device between the twohalves. A rough road sensor detects when the vehicle is travelling overa rough road and, in response, the two halves of the anti-roll bar arede-coupled from each other so as to increase the ease of articulation.On smooth roads the two halves are coupled together again so as toincrease roll stiffness.

A similar conflict arises with dampers which are usually required toprovide a relatively high level of damping for on-road use, but areduced level of damping to increase ease of articulation during offroad use. It is well known to address this problem using switchabledampers which have a damping rate which can be varied to suit thecurrent driving conditions.

Whilst these systems do address the problem, there is always a need forbetter control of such systems.

SUMMARY OF THE INVENTION

The present invention provides a vehicle suspension system forconnecting at least one pair of wheels to opposite sides of a vehiclebody, the system comprising a suspension associated with said at leastone pair of wheels and arranged to allow articulation movement of saidat least one pair of wheels relative to the body and to provideadjustable resistance to said articulation movement, sensors arranged tomeasure movement of the suspension, and a controller arranged to monitorsaid movement and to determine therefrom the level of said articulationmovement and to reduce said resistance if said articulation movementreaches a predetermined level.

Preferably the adjustable resistance to articulation movement is anadjustable articulation stiffness. However, it could also be anadjustable damping rate provided by dampers which either damp allvertical movements of the individual wheels, or damp specificallyarticulation movements.

The measured articulation can be single axle articulation, that is thedifference in ride height between the two front wheels, or thedifference in ride height between the rear two wheels of a typicalfour-wheeled vehicle. Alternatively the measured articulation may becross articulation, that is the difference between the articulation ofthe front wheels, and the articulation of the rear wheels.

Preferably the sensors are arranged to measure the ride height of eachof said at least one pair of wheels and the controller is arranged todetermine the level of articulation movement of said at least one pairof wheels from the measured ride heights.

Preferably the controller is arranged to determine a level of crossarticulation movement between two pairs of the wheels, each pair beingon opposite sides of the body, and to reduce said resistance if saidcross articulation movement reaches a predetermined level.

The sensors may be arranged to measure the ride heights of each of bothpairs of wheels, and the controller arranged to determine the level ofcross articulation movement from the measured ride heights.

Preferably the controller is arranged to calculate a running measure ofthe level of articulation movement and to reduce said resistance if therunning measure reaches a predetermined level.

Preferably the running measure is arranged to increase during periodswhen said level of articulation movement is high and to decrease duringperiods when said level of articulation movement is low.

Preferably the system further comprises a vehicle speed sensor arrangedto measure a travelling speed of the vehicle, and the running measureincludes a speed dependent offset arranged to cause it to reduce whenthe travelling speed of the vehicle is high.

Preferably the controller is arranged to increase the level of saidresistance when the running measure falls below a predetermined level.

Preferably the controller is arranged to measure an instantaneousarticulation displacement of the wheels, and running measure is arunning average of the articulation displacement.

Preferably the controller includes a low pass filter and is arranged,when determining the level of said articulation movements, to filter outarticulation movements of a frequency higher than a predetermined limitfrequency.

Preferably the body has a natural frequency of vibration on thesuspension and said limit frequency is of the order of said naturalfrequency of vibration, which is desirably of the order of 2 to 3 Hz.

Preferably the articulation sensors are arranged to measure theinstantaneous articulation displacement of the wheels and to reduce saidresistance only when said displacement is less than a predeterminedlimit.

Preferably the controller is arranged to detect when the vehicle istravelling on a side slope, and, in response to detection of a sideslope, to increase said resistance.

The present invention further provides a vehicle suspension system forconnecting at least one pair of wheels to opposite sides of a vehiclebody, the system comprising a suspension associated with said at leastone pair of wheels and arranged to allow articulation movement of saidat least one pair of wheels relative to the body and to provideadjustable resistance to said articulation, a side slope detectorarranged to detect when the vehicle is travelling on a side slope, and acontroller operably connected to the suspension and the side slopedetector and arranged to increase said resistance in response todetection of a side slope.

Where said resistance is adjustable between a high level and a lowlevel, the controller is preferably arranged to detect when the vehicleis travelling on a side slope, and, if a side slope is detected when theresistance is at the low level, to adjust it to the high level.

Preferably the system further comprises a plurality of sensors arrangedto send signals to the controller and the controller is arranged to usethe signals to detect when the vehicle is travelling on a side slope.

The present invention further provides a vehicle suspension system forconnecting at least one pair of wheels to opposite sides of a vehiclebody, the system comprising a suspension associated with said at leastone pair of wheels and arranged to allow articulation movement of saidat least one pair of wheels relative to the body and to provideadjustable resistance to said articulation, a plurality of sensors, anda controller operably connected to the suspension and the sensorswherein the controller is arranged to receive signals from the sensors,to determine from said signals when the vehicle is travelling on a sideslope, and to increase said resistance in response to detection of aside slopes.

Preferably said resistance is adjustable between a high level and a lowlevel and the controller can be shut down and started up and isarranged: to measure an instantaneous articulation displacement of thewheels, on shut down to store the instantaneous articulationdisplacement of the wheels and the instantaneous level of saidresistance, and on start up, if the instantaneous level of saidresistance on shut down was the low level, to compare the instantaneousdisplacement with the stored displacement and, if they are substantiallythe same, to return the resistance to the low level.

The present invention further provides a vehicle suspension system forconnecting at least one pair of wheels to opposite sides of a vehiclebody, the system comprising a suspension associated with said at leastone pair of wheels and arranged to allow articulation movement of saidat least one pair of wheels relative to the body and to provideresistance to said articulation movement which resistance is adjustablebetween a high level and a low level, and a controller for adjustingsaid resistance wherein the controller can be shut down and started upand is arranged: to measure an instantaneous displacement of the wheels,on shut down to store the instantaneous displacement of the wheels andthe instantaneous level of said resistance, and on start up, if theinstantaneous level of said resistance on shut down was the low level,to compare the instantaneous displacement with the stored displacementand, if they are substantially the same, to return the resistance to thelow level.

Preferably the controller is arranged, if said instantaneousdisplacement and said stored displacement are not substantially thesame, to measure an instantaneous articulation displacement of thewheels and, if it is below a predetermined threshold to return theresistance to the low level.

Preferably the controller is arranged, if said instantaneousarticulation is above said predetermined threshold, to delay returningof said resistance to said low level until the instantaneousarticulation next falls to said predetermined threshold.

The suspension may include fluid filled suspension units associated withthe wheels, with an adjustable fluid interconnection provided betweenthe units, the controller being arranged to reduce said resistance byadjusting the interconnection. The fluid may be air or hydraulic fluid.

Alternatively the suspension may include a split anti-roll bar havingtwo halves which can be de-coupled from each other to reduce saidresistance.

The present invention further provides a vehicle suspension system forconnecting a pair of wheels to opposite sides of a vehicle body, thesystem comprising a suspension associated with said wheels and arrangedto allow articulation movement of wheels relative to the body and toprovide adjustable resistance to said articulation movement, ride heightsensors arranged to measure the ride height or each of the wheels, and acontroller arranged to monitor the measured ride heights and todetermine therefrom a level of said articulation movement and to reducesaid resistance if said articulation movement reaches a predeterminedlevel.

The present invention still further provides a vehicle suspension systemfor connecting two pairs of wheels to opposite sides of a vehicle body,the system comprising a suspension associated with said wheels andarranged to allow articulation movement of each of said pairs of wheelsrelative to the body and to provide adjustable resistance to saidarticulation movement, ride height sensors arranged to measure the rideheight of each of the wheels, and a controller arranged to monitor themeasured ride heights and to determine therefrom a level of crossarticulation movement of the wheels, and to reduce said resistance ifsaid level of cross articulation movement reaches a predetermined level.

Preferred embodiments of the present invention will now be described byway of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a vehicle including asuspension system according to a first embodiment of the invention, and

FIG. 2 is a diagrammatic representation of a vehicle including asuspension system according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A vehicle has two front wheels 10, 12 and two rear wheels 14, 16, thefront pair being mounted on a front beam axle 18 and the rear pair beingmounted on a rear beam axle 20. The axles 18, 20 are connected to thevehicle body 22 by an air suspension system which includes four airsprings 24, 25, 26, 27, one at each end of each axle 18, 20. Each airspring therefore controls the ride height at a respective one of thewheels 10, 12, 14, 16, that is the height of the body relative towheels. The air springs are each pneumatically connected to a valveblock 28 which controls the flow of air from a compressor 30 to thesprings, and from the springs to atmosphere. The valve block 28 andcompressor 30 are controlled by a control unit 32. A pneumaticinterconnection 34 is also provided between the front two air springs24, 25, and another 36 between the rear two 26, 27. Each interconnection34, 36 can be opened and closed by a respective cross-link valve 38, 40.

The control unit 32 is connected to ride height sensors 42, 43, 44, 45each associated with a respective one of the wheels 10, 12, 14, 16 andwith a vehicle speed sensor 46 which could comprise one or more wheelspeed sensors from an anti-lock braking system. The ride height sensors42, 43, 44, 45 measure the ride height of the wheels by measuring therelative position of a part of the suspension system, such as the endsof the axles 18,20, and the body 22.

Under normal on-road driving conditions the cross-link valves 38, 40 arekept closed and the control unit 32 controls the flow of air into andout of the air springs 24, 25, 26, 27 on the two sides of the vehicle soas to control vehicle roll. However, if it detects, as described below,that the vehicle is travelling over rough terrain where low resistanceto articulation will be required to maintain traction and optimize theload distribution between the wheels 10, 12, 14, 16, it opens the crosslink valves, This decreases the articulation stifffiesses of the frontand rear axles, and therefore decreases the cross articulation stiffnessof the suspension.

Rough terrain generally produces high levels of cross articulationmovement of the wheels at relatively low frequencies. Therefore thesystem needs to monitor for these high levels of movement either incross articulation, or in single axle articulation.

A first method of monitoring for and detecting conditions when thecross-linking valves 38, 40 should be opened will now be described. Thismethod relies on the fact that, for a moving vehicle, if the averagearticulation displacement is high, then this implies that a high levelof articulation movement is occurring. This is particularly true ofcross articulation since the cross articulation displacement cannot stayconstant in a moving vehicle. It is also generally true of single axlearticulation because high levels of roll are unlikely to be sustainedfor long periods except when cornering at high speeds, or when thevehicle is on a side slope. Methods of dealing with these circumstanceswill be described below.

Firstly the control unit uses the signals from the ride height sensors42, 43, 44, 45 to calculate a the instantaneous level of articulation ofthe front wheels, and that of the rear wheels. To measure theinstantaneous articulation displacement A_(f) of the front wheels thecontrol unit subtracts the ride height at the front left wheel H_(fl)from that at the front right wheel H_(fr), using the equation:

 A _(r) =R _(fr) −H _(fl)

Similarly the instantaneous articulation displacement of the rear wheelsis calculated by subtracting the ride height at the rear left wheelH_(rl) from that at the rear right wheel H_(rr), using the equation:

A _(r) =H _(rr) −H _(rl)

Each articulation measure A_(f) and A_(r) therefore has a magnitude,which will be zero when the two wheels are at the same ride height, anda sign which can be positive or negative depending on whether thearticulation is to the left or to the right. Therefore at any instantthe instantaneous level of cross articulation displacement A_(cross) canbe determined by subtracting the rear articulation displacement A_(r)from the front articulation displacement A_(f), using the equation:

A _(cross) =A _(f) −A _(r)

Because this value can be positive or negative, and will be zero whenthe ride heights of all the wheels are equal or the vehicle isexperiencing pure roll or pure pitch movement, the modulus of it is thentaken and filtered using a low pass filter. This cuts out the parts ofthe signal which are due to high frequency articulation movements, orvibrations, from small scale unevenness in the road surface and producesa signal A_(cross,LPF) indicative of the cross articulation produded bylarger scale unevenness in the surface being driven on. The frequenciesof interest are those of the order of the natural frequency of thevehicle body or lower. That natural frequency can the natural frequencyof vibration of the body in bounce or pitch or roll movement relative tothe wheels, since these frequencies are all generally similar. The lowpass filter therefore cuts out frequencies above a limit which willgenerally be in the range from 2 to 3 Hz. The level of crossarticulation movement is then measured by producing a running measure,or running average of the articulation displacement in the form of anintegral Z_(cross) of (A_(cross,LPF)−X)k, i.e.Cross_Articulation_level  Z_(cross) = ∫₀^(∞)(A_(cross, LPF) − X) * k  

where X is a speed dependent offset which increases with vehicle speedand k is a scaling factor. It will be appreciated that at higher speedswith low levels of cross articulation the integral Z_(cross) will fallin value with time, but at lower speeds with higher levels ofarticulation Z_(cross) will rise with time. When this integral Z_(cross)is higher than a predetermined threshold value Z_(cross, limit), thisindicates that the vehicle is travelling on a rough road surface, Thecontrol unit therefore checks whether the modulus of the crossarticulation displacement A_(cross), is smaller than a predeterminedvalue A_(cross, limit). If it is, or otherwise if it falls to that valuewhile the integral Z_(cross) is still high indicating a rough road, thenthe cross linking valves are opened to allow the wheels to articulatemore easily. When this integral Z_(cross) falls to below the thresholdvalue Z_(cross, limit), this indicates that the vehicle is no longertravelling on a rough road surface. The control unit checks whether themodulus of the cross articulation displacement A_(cross), is smallerthan the predetermined threshold value A_(cross, limit). If it is, orotherwise if it falls to that threshold value while the integralZ_(cross) is still low indicating a smooth road, then the cross linkingvalves are closed again to make the suspension stiffer in roll. Thevalves are only opened and closed at low levels of cross articulationdisplacement to ensure a smooth transition from the closed to the openstates, and back. If this were not done on the transition from theclosed state to the open state then sudden changes of attitude and rideheight of the vehicle could occur as the air is suddenly able to flowbetween the air springs. If it were not done on the transition from theopen state to the closed state, then the distribution of air between thetwo sides of the vehicle could be temporarily held in an uneven state.The value of A_(cross, limit) can be chosen to suit the particularvehicle and the level of refinement required, but in this particularembodiment is 20 mm.

It will be appreciated that the system can be tuned by varying the rateat which the offset X varies with speed and by varying the size of thescaling factor k. For example, the offset X will generally be low at lowspeeds so that the integral Z_(cross) can build up quickly resulting inearly detection of articulation. It can then be made to increase veryrapidly at higher speeds so that the cross linking valves will always beclosed above a certain speed. The speed selected will depend on theparticular vehicle and the handling characteristics required, but couldbe 40 kph for example. With a small offset the decay of the integralZ_(cross) will be slow, which can be helpful to prevent switching of thevalves to the closed position when the vehicle is only on a shortstretch of smooth road.

When the vehicle is stopped and the system shut down, the status of thecross link valves 38, 40 is checked and stored in non-volatile memory 32a in the control unit. The ride heights of each of the wheels is alsostored, as is the current value of the cross articulation displacementintegral Z_(cross). Then when the system is started up again, if thevalves had been open on shut down, then it is assumed that the vehicleis still on rough ground, and that they therefore need to be openedagain on start up. In this way the system can carry on essentially as ifit had not been shut down. However, before they are opened, the currentride heights are compated with those stored in memory. If they are thesame, then the cross link valves 38, 40 are opened immediately. However,if the ride heights have changed, then the pressure of air in the airsprings may have changed, and opening the valves immediately could causesome undesirable suspension travel as the pressures in the air springsequalised. Therefore the instantaneous level of cross articulationdisplacement is checked, and the valves 38, 40 only opened if it isbelow the threshold value A_(cross, limit). If it is above thatthreshold value, then opening of the valves is delayed until it nextfalls to that threshold value, at which point the valves are opened.

As mentioned above, the system also needs to deal with situations wherethe vehicle is driving on a side slope. With the first embodiment, whichuses a measure of cross articulation to control the level of resistanceto articulation, a side slope will not interfere with the detection ofrough terrain. However if the vehicle is travelling on a side slope,opening the cross link valves 38, 40 and thereby reducing the rollstiffness of the vehicle will allow the vehicle body 22 to roll towardsthe downhill side of the vehicle. This can reduce the lateral stabilityof the vehicle. Therefore the control unit 32 is arranged to provide ameasure of the side slope and, if that measure exceeds a certainthreshold or limit, implying that the vehicle is on a side slope of atleast a certain steepness or angle, to close one or both of the crosslink valves 38, 40, irrespective of the levels of articulation movementbeing measured, thereby increasing the roll stiffness. The degree ofside slope at which it is desirable to close permanently the cross linkvalves and increase the roll stiffness will depend on the particularvehicle, but in this particular embodiment corresponds to a lateralacceleration of 0.2 g.

The method of detection of a side slope is not critical to thisinvention, and will not be described in detail. However a preferredmethod is disclosed in our patent application WO 99/64262, the entirecontents of which are incorporated herein by reference. Essentially thatmethod entails measuring the lateral acceleration of the vehicle,calculating a maximum lateral acceleration which would be expected as aresult of cornering, based on the vehicle speed and an estimate of theminimum turning circle diameter, and comparing the measured lateralacceleration with the expected cornering acceleration to provide ameasure of the degree of side slope. Other methods of detecting andmeasuring side slope which could also be used use measurement of thewheel speeds of each of the vehicle wheels to give a measure of vehiclespeed and yaw rate. These then allow a calculation of corneringacceleration, which can be compared with a measured lateral accelerationto determine the side slope. As a further modification to this methodvehicle speed and steering angle can be used to determine the corneringacceleration.

The choice of which of the cross link valves 38, 40 should be closedwill depend on the particular vehicle. Closing both together willclearly give maximum roll stiffness, but also increase significantly theresistance to articulation. It might therefore be beneficial to closeone of the cross link valves 38, 40, but leave the other open for atleast side slopes up to a certain predetermined angle, thereby giving adegree of roll control but still allowing a reasonable level of crossarticulation, but to close both on extreme side slopes of greater than ahigher angle.

In a second embodiment of the invention the cross linking valves 38, 40can each be opened and closed independently of the other, on the basissolely of the level of articulation of the respective axle 18, 20. Inthis case the two articulation displacements A_(f) and A_(r) aremeasured as above and the modulus of each taken and low pass filtered. Arunning integral Z_(x) for each is then taken of the same form as thatof the cross articulation described above, i.e.Articulation_level  Z_(x) = ∫₀^(∞)(Articulation_(x, LPF) − X) * k

where X is a speed dependent offset which increases with vehicle speedand k is a scaling factor, and x is either f for the front articulationor r for the rear articulation. The cross link valve 38, 40 for eachaxle is then controlled solely on the basis of the running articulationdisplacement measure Z_(x) for that axle, being opened when it risesabove a threshold value, and closed again when it falls below thatvalue.

This method has the advantage that each axle can be controlled to suitthe conditions in which it is operating. Also it gives the opportunityfor the two axles to have different offsets or scaling factors whichmight be advantageous for example where the loading of the front andrear axles is very different, or where the front and rear suspensionsare different.

The side slope strategy described in relation to the first embodimentcan also be applied to this embodiment. However for side slopes where itis desired to have one valve open and the other closed, the system needsto be able to respond to the fact that either one of the valves may beopen while the other is closed. For example it could be arranged toensure simply that, on less extreme side slopes at least one of thevalves is closed, on and more extreme side slopes both of them areclosed.

Referring to FIG. 2, in a further embodiment of the invention, a vehiclehas an independent suspension including suspension arms 118 a, 118 b,120 a, 120 b by means of which the front wheels 110, 112 and the rearwheels 114, 116 are connected to the vehicle body 122. A split anti-rollbar 136 is connected between the two rear wheels 114, 116. Thisanti-roll bar has two halves 136 a, 136 b with a decoupling device 140between them, which may for example take the form of that shown in U.S.Pat. No. 4,796,911. This allows the two halves of the anti-roll bar tobe locked together so that it acts as a conventional one-piece anti-rollbar resisting articulation of the rear wheels, or decoupled so that itdoes not significantly resist such articulation. A similar splitanti-roll bar could also be connected between the front wheels 110, 112.An electronic control unit 132 controls the decoupling device 140 basedon inputs from ride height sensors 142, 143, 144, 145 associated withthe four wheels, and a vehicle speed sensor 146. The control unitoperates in the same way as that of FIG. 1, keeping a running measure ofthe cross articulation of the front and rear wheels, or of thearticulation of each axle independently, and then locking the two halvesof the anti-roll bar (or bars) together when the articulation integral Zis below a threshold value, and de-coupling the two halves when it isabove the threshold.

It will also be appreciated that there are other forms of suspension inwhich the resistance to single axle articulation or cross articulationcan be varied, such as interconnected hydraulic systems, to which thisinvention is equally applicable.

We claim:
 1. A vehicle suspension system for connecting at least onepair of wheels to opposite sides of a vehicle body, the systemcomprising a suspension associated with said at least one pair of wheelsand arranged to allow articulation movement of said at least one pair ofwheels relative to the body and to provide resistance to saidarticulation movement which resistance is adjustable between a highlevel and a low level, and a controller for adjusting said resistancewherein the controller can be shut down and started up and is arranged:to measure an instantaneous displacement of the wheels, on shut down tostore the instantaneous displacement of the wheels and the instantaneouslevel of said resistance, and on start up, if the instantaneous level ofsaid resistance on shut down was the low level, to compare theinstantaneous displacement with the stored displacement and, if they aresubstantially the same, to return the resistance to the low level.
 2. Asystem according to claim 1 wherein the controller is arranged, if saidinstantaneous displacement and said stored displacement are notsubstantially the same, to measure an instantaneous articulationdisplacement of the wheels and, if it is below a predetermined thresholdto return the resistance to the low level.
 3. A system according toclaim 2 wherein the controller is arranged, if said instantaneousarticulation is above said predetermined threshold, to delay returningof said resistance to said low level until the instantaneousarticulation next falls to said predetermined threshold.
 4. A vehiclesuspension system for connecting at least one pair of wheels to oppositesides of a vehicle body, the system comprising: a suspension associatedwith the at least one pair of wheels and arranged to allow articulationmovement of the at least one pair of wheels relative to the body and toprovide adjustable resistance to the articulation movement; a pluralityof sensors arranged to measure movement of the suspension and; acontroller arranged to monitor said movement of the suspension and todetermine therefrom a difference in ride height between the at least onepair of wheels thereby to determine a level of said articulationmovement over a period, and to reduce the resistance if said levelreaches a predetermined level, wherein the controller is arranged tomeasure the articulation of a first pair of the wheels and to measurethe articulation of a second pair of the wheels and to measure thedifference between the articulation of the first and second pairs ofwheels thereby to measure a cross articulation movement, and to reducethe resistance if the cross articulation movement reaches apredetermined level.
 5. A system according to claim 4, wherein thesensors are arranged to measure the ride heights of each of both pairsof wheels, and the controller is arranged to determined the level ofcross articulation movement from the measured ride heights.
 6. A systemaccording to claim 4, wherein the controller is arranged to calculate arunning measure of the level of articulation movement and to reduce theresistance if the running measure reaches a predetermined level.
 7. Asystem according to claim 6, wherein the running measure is arranged toincrease during periods when the level of articulation movement is highand to decrease during periods when the level of articulation movementis low.
 8. A system according to claim 6, wherein the controller isarranged to increase the level of the resistance when the runningmeasure falls below a predetermined level.
 9. A system according toclaim 6, wherein the controller is arranged to measure an instantaneousarticulation displacement of the wheels, and running measure is arunning average of the articulation displacement.
 10. A system accordingto claim 4, wherein the controller includes a low pass filter and isarranged, when determining the level of the articulation movements, tofilter out articulation movements of a frequency higher than apredetermined limit frequency.
 11. A system according to claim 10,wherein the body has a natural frequency of vibration on the suspensionand the limit frequency is of the order of the natural frequency ofvibration.
 12. A system according to claim 10, wherein the limitfrequency is on the order of 2 to 3 Hz.
 13. A system according to claim4, wherein the controller is arranged to detect when the vehicle istraveling on a side slope, and, in response to detection of a sideslope, to increase the resistance.
 14. A system according to claim 4,wherein the resistance is adjustable between a high level and a lowlevel, and the controller is arranged to detect when the vehicle istraveling on a side slope, and, if a side slope is detected when theresistance is at the low level, to adjust it to the high level.
 15. Asystem according to claim 14 further comprising the plurality of sensorsarranged to send signals to the controller and the controller isarranged to use the signals to detect when the vehicle is traveling on aside slope.
 16. A system according to claim 4 further comprising a sideslope detector arranged to detect when the vehicle is traveling on aside slope, and a controller operably connected to the suspension andthe side slope detector and arranged to increase the resistance inresponse to detection of a side slope.
 17. A system according to claim16, wherein the resistance is adjustable between a high level and a lowlevel, and the controller is arranged, in response to detection of aside slope when the resistance is at the low level, to adjust theresistance to the high level.
 18. A system according to claim 4 furthercomprising the plurality of sensors, wherein the controller is arrangedto receive signals from the sensors, to determine from the signals whenthe vehicle is traveling on a side slope, and to increase the resistancein response to detection of a side slope.
 19. A system according toclaim 18, wherein the resistance is adjustable between a high level anda low level, and the controller is arranged, in response to detection ofa side slope when the resistance is at the low level, to adjust theresistance to the high level.
 20. A system according to claim 4, whereinthe suspension includes fluid filled suspension units associated withthe wheels, an adjustable fluid interconnection is provided between theunits and the controller is arranged to reduce the resistance byadjusting the interconnection.
 21. A system according to claim 20,wherein the fluid is air.
 22. A system according to claim 4, wherein thesuspension includes a split anti-roll bar having two halves which can bede-coupled from each other to reduce the resistance.
 23. A vehiclesuspension system for connecting at least one pair of wheels to oppositesides of a vehicle body, the system comprising: a suspension associatedwith the at least one pair of wheels and arranged to allow articulationmovement of the at least one pair of wheels relative to the body and toprovide adjustable resistance to the articulation movement; a pluralityof sensors arranged to measure movement of the suspension and; acontroller arranged to monitor the movement and to determine therefromthe level of the articulation movement and to reduce the resistance ifthe articulation movement reaches a predetermined level wherein thecontrol means is arranged to measure a difference in ride height betweenthe at least one pair of wheels and to determine the level ofarticulation movement of the at least one pair of wheels from themeasured difference in ride heights; the controller is arranged tocalculate a running measure of the level of articulation movement and toreduce the resistance if the running measure reaches a predeterminedlevel and the running measure is arranged to increase during periodswhen the level of articulation movement is high and to decrease duringperiods when the level of articulation movement is low; and wherein avehicle speed sensor is arranged to measure a traveling speed of thevehicle and the running measure includes a speed dependent offsetarranged to cause it to reduce when the traveling speed of the vehicleis high.
 24. A vehicle suspension system for connecting at least onepair of wheels to opposite sides of a vehicle body, the systemcomprising: a suspension associated with the at least one pair of wheelsand arranged to allow articulation movement of the at least one pair ofwheels relative to the body and to provide adjustable resistance to thearticulation movement; a plurality of sensors arranged to measuremovement of the suspension; and a controller arranged to monitor themovement and to determine therefrom the level of the articulationmovement and to reduce the resistance if the articulation movementreaches a predetermined level wherein the articulation sensors arearranged to measure the instantaneous articulation displacement of thewheels and to reduce the resistance only when the displacement is lessthan a predetermined limit.
 25. A vehicle suspension system forconnecting at least one pair of wheels to opposite sides of a vehiclebody, the system comprising: a suspension associated with the at leastone pair of wheels and arranged to allow articulation movement of the atleast one pair of wheels relative to the body and to provide adjustableresistance to the articulation movement; a plurality of sensors arrangedto measure movement of the suspension; and a controller arranged tomonitor the movement and to determine therefrom the level of thearticulation movement and to reduce the resistance if the articulationmovement reaches a predetermined level, wherein the resistance isadjustable between a high level and a low level and the controller canbe shut down and started up and is arranged: to measure an instantaneousarticulation displacement of the wheels, on shut down to store theinstantaneous articulation displacement of the wheels and theinstantaneous level of the resistance, and on start up, if theinstantaneous level of the resistance on shut down was the low level, tocompare the instantaneous displacement with the stored displacement and,if they are substantially the same, to return the resistance to the lowlevel.